Method for producing drawn coated metals and use of said metals in the form of a current differentiator for electrochemical components

ABSTRACT

The present invention pertains to a process for manufacturing expanded metal provided with a coating, characterized in that the coating is applied to a closed metal foil and this is converted into expanded metal only after the coating. In particular, the coating may be a coating that improves the adhesiveness of the expanded metal to an electrode material and/or the electron conductivity on the surface of the expanded metal. Such expanded metals can be advantageously used as current collectors in or for an anode foil or in or for a cathode foil, e.g., in an electrochemical cell, especially in a battery.

The present invention pertains to a process for manufacturing coatedexpanded metals, which are suitable for use, among other things, ascurrent collectors in electrochemical components, especially innonaqueous electrochemical cells.

Typical representatives of nonaqueous electrochemical cells are lithiumbatteries. These have been known in various embodiments for a long timeand have been described several times. The design of these cells is asfollows: An anode, either one consisting of lithium metal or graphite,is arranged opposite a cathode, usually a stable lithium interstitialcompound. The two electrodes are separated by a separator. The completesystem is interspersed by an electrolyte, which establishes the ionicconductivity for lithium ions. They are formed, as a rule, by a lithiumsalt dissolved in one or more organic solvents. Lithium ions move to andfro between the electrodes during charging and discharging.

To avoid the problem of unbound electrolyte liquids inside the batterybody, successful attempts were made at making the electrodes and theseparator in the form of foils. These foils are characterized either bya high microporosity, in which the liquid electrolyte is immobilized, orby the addition of suitable polymers, which form a gel with the liquid.

Besides the electrochemical components described, such componentsrequire current collectors to collect and drain off the electron currentand make is usable for the user. The current collectors are metals,which are introduced either as foils or as expanded metals. They shallmeet a number of conditions, namely, (a) they must be electrochemicallystable against corrosion, (b) they shall have good contact with theparticular electrodes to ensure a low contact resistance, (c) they shallhave a low weight in order to guarantee high energy densities, and (d)they shall have favorable elastic properties in order to compensatevariations in volume during the intercalation and de-intercalation oflithium ions in the electrodes during the operation. Aluminum andgraphite have proved to be suitable for use as metals that are stable inthe electrochemical environment of the battery for the system usedcommonly with lithium cobalt oxide as the cathode material and graphiteon the anode side. However, the process being described here is notlimited to these metals.

The manufacture of expanded metals shall be briefly explained below. Theprocess is schematically shown in FIG. 1. Metal foils (1) of a suitablethickness are provided with a punched pattern in a punching tool (2) andthen stretched (3). The geometric data of the expanded metal, such asthe width of the web, the opening diagonals and the percentage of openarea are set by designing the punched pattern and the rate ofstretching.

The advantages of the use of expanded metals as current collectors areobvious: Compared to foils, they have an open-pore structure, so thatthe weight of the current collectors can be reduced, which entailsadvantages in the gravimetric energy density. In addition, the expandedmetal is elastic in such a way that it can follow the changes in volumeduring the intercalation and de-intercalation of lithium in theelectrodes, without delamination taking place. Delamination would inturn reduce the cycle life of the batteries.

Another advantage is achieved in terms of production engineering whenexpanded metal is used in lithium-polymer batteries. This type ofbattery is usually manufactured as follows: The electrodes are eitherdeposited directly on metal foils (see U.S. Pat. No. 6,306,215) or arelaminated on the current collector by lamination under pressure andoptionally under the action of temperature (DE 199 52 335). A firmcomposite of the three different foils for the anode with the currentcollector, the separator and the cathode with the current collector isthen prepared in a second step by lamination or by the windingtechnique. This composite is then impregnated with electrolyte liquid.The electrolyte liquid must be distributed uniformly in the completefoil composite. This is achieved essentially by capillary forces. Thisprocess is frequently also supported by the additional formation ofpores, which are produced by adding a plasticizing agent to theelectrode and separator materials, which is again removed by means of asolvent after the components have been laminated together. Closed metalfoils make difficult the penetration of the liquid electrolyte into thebattery body, i.e., they prolong the duration of the process. The use ofan open-mesh expanded metal therefore offers considerable advantages interms of process engineering.

To achieve good adhesion and low contact resistance at the interfacebetween the metallic current collector and the laminated electrode, itproved to be advantageous to coat the current collector with a thinlayer of adhesion promoter before it is connected to the electrode, andit is especially advantageous if the said adhesion promoter layer has anelectron conductivity. A number of proposals have been made in the stateof the art concerning the technique to be used to apply such adhesionpromoters and their suitable compositions. They comprise the coating ofcurrent collectors made of expanded metals or of other perforatedcollectors (nettings, grids) in liquids or pastes by spin coating,dipping or coating (see U.S. Pat. No. 6,306,215 and U.S. Pat. No.5,824,120), the application of a layer that contains carbon powder andan adhesiveness-improving polymer, by means of electrostatic forces(U.S. Pat. No. 5,542,163), or a plasma polymerization method, by which alayer of an electrically conductive, polymeric, adhesion-improvingmaterial is applied to the current collector (see U.S. Pat. No.6,007,588). All the documents mentioned describe electrochemical cellsfrom electrode materials with a plasticizing agent, which is againwashed out of the cells after the individual components (electrode andseparator layers or foils, current collector) have been laminatedtogether in order to generate pore volume for the electrolyte liquid, aswas described above. It is also possible to apply the coating byapplying a suitable suspension by means of printing methods.Corresponding suspensions are commercially available. Methods based onthe use of printing rollers, for example, reverse roll coating, areused. The suspensions consist, as a rule, of a carbon/polymer mixture ina suitable solvent, such as water.

Considerable problems arise in practical application when coatingsolutions are applied according to methods used in the printing trade.The layer thickness of the suspensions applied are usually betweenapprox. 1 μm and approx. 20 μm. Insufficient wetting is invariablyobserved in the printing roller process, which leads to nonuniformdistribution of the suspension on the metal. As a result, the contactresistance increases or contact is even lost between the currentcollector and the electrode at the poorly coated sites during theoperation of electrochemical components equipped with expanded metalscoated in this manner, e.g., batteries, and this has disadvantageousconsequences for the service life of the components. However, the use ofthicker adhesion promoter layers is ruled out because of the undesiredreduction of the energy density that is associated with this.

Moreover, problems arise in terms of the hardware when printing methodsare used to apply the adhesion promoter layers. To guarantee the preciseguiding of the foils during coating, guiding of the metal foil overseveral deflecting rollers is necessary. In addition, the foil must bekept under a certain mechanical tension during its run through thecoating machine. While this does not cause any technical problem in caseof the use of closed metal foils, for example, foils made of aluminum orcopper in case of foil thicknesses as low as approx. 10 μm to 15 μm,expanded metals tend to tear even under low tensile stresses. This isespecially critical in case of aluminum expanded metals. Typicalthicknesses of expanded metals are 50 μm or even less. Massive yieldproblems arise due to tearing even on machines designed especially forcoating expanded metals. However, a changeover to thicker currentcollectors to improve the reliability of the process is just asimpossible, because of an undesired reduction of the energy density, ascoating with an excessively thick layer.

The object of the present invention is to provide a process formanufacturing coated expanded metals, which leads to improved yields andwith which the top side and the underside of thin expanded metals canalso be coated with a sufficiently thin layer of conductive adhesionpromoters.

The said object is accomplished by a process in which a closed metalfoil is first coated and this is then converted into expanded metal.This offers the advantage that the coating is applied to a mechanicallysubstantially more stable metal foil, so that a product possessing thenecessary properties can be manufactured with a high yield and theamount of rejects can be greatly reduced. Coating may be performed onone side or on both sides. It was quite surprising to find with thisprocedure that the coating applied to the foil does not flake off duringstretching. This was not to be expected at all, because it was notpossible to assume that it would be sufficiently elastic and, moreover,possess such a good adhesion that the deformation of the metal lyingunder the coating does not lead to separation of the coating.

It was equally fully surprising that another advantage of the presentinvention was able to be observed, namely, that the service life of thepunching knives increases during the manufacture of the expanded metal.This could be due to the fact that usual adhesion promoters aresuspensions containing graphite, which act as lubricants for the knivesduring the punching operation and thus contribute to the prolongation oftheir service life.

The present invention is explained in the attached drawings, in which

FIG. 1 shows the sequence of a laminate suitable for use for a battery,

FIG. 2 shows the top view of such a laminate,

FIG. 3 shows a schematic view of the manufacture of the expanded metal,

FIG. 4 shows a diagram showing the relative capacity of a batteryprovided with an expanded metal manufactured according to the presentinvention as a collector, and

FIG. 5 shows the relative capacity of a battery with an expanded metalmanufactured according to the present invention as a collector comparedto the capacity of a battery with a collector manufactured in the usualmanner.

It is especially favorable if the metal foil is subjected to a coronadischarge surface treatment already before the coating operation,because this measure leads to a further improvement in the adhesion ofthe coating on the expanded metal.

It is frequently preferred to stretch the metal during the expansion atmost only to the extent that the short diagonal will have a length ofabout 1 mm and the long diagonal will have a length of about 2 mmbecause, depending on the flexibility of the coating materials used, thecoating may separate in some cases when a greater stretching is carriedout.

All the materials with which the desired properties that the expandedmetal needs for its later use can be obtained are suitable for coatingthe metal foils that will subsequently be subjected to the expansionprocess. These are above all good adhesion to the electrodes as well asgood electric conductivity in the case of expanded metals used ascurrent collectors. However, it should be clear that the processaccording to the present invention is not limited to the manufacture ofcoated expanded metals for current collectors. It can rather be usedwherever thin expanded metals with sensitive, thin coatings are to beused and it is not necessary that the openings generated during thepunching and stretching also be coated laterally.

For example, materials such as graphite or other suitable carbonmaterials as well as adhesion-improving organic polymers shall bementioned as suitable materials for coatings with good adhesion and goodelectric conductivity. The carbon materials may be applied in a binder,e.g., an organic polymer suspension, which binder can subsequently bedried, (after)cured or subjected to an additional polymerization on thesurface. One example is EB-012 from the firm of Acheson, U.S.A., agraphite suspension, which contains a thermoplastic binder. Otherexamples are suspensions containing silver instead of graphite. Thebinders may be, e.g., epoxy resins, thermoplastics, duromers, vinylresins, cellulose or fluoroelastomers. However, it is also possible touse other suitable materials instead of graphite suspensions if theyimpart the said properties, for example, electrically conductive organicpolymers such as polyvinylpyrrolidone. Furthermore, polymer suspensionsthat are graphitized after the application to the metal are well suitedfor use as a coating.

The process according to the present invention was found in light of thepoor quality of expanded metals coated according to the printing method.However, it is not limited to specific coating techniques. Instead ofapplication according to the printing method, it is also possible touse, e.g., spin coating, roller coating, application with a doctorblade, dip coating, electrostatic application (powder coating) or theplasma method, as they are known, among other things, from theabove-mentioned state of the art.

The expanded metals manufactured according to the present inventiondiffer from the conventional ones by the fact that their openings,produced during the punching and stretching, are not coated laterally.However, this is of no disadvantage for their use as current collectors.

The expanded metals that are or can be manufactured according to thepresent invention are especially suitable, among other things, for usein electrochemical cells during the manufacture of which the addition ofa plasticizer, which would have to be removed again in a subsequentwashing process, to the electrode materials and/or the separator toproduce a porosity necessary for taking up the liquid electrolyte isavoided, because this manufacturing process, which is described in theU.S. Pat. Nos. 5,456,000 and 6,063,519, requires, as an additionalrequirement on the adhesion promoter layer, that this layer bechemically stable in respect to the wash liquid. Partial separation ofthe electrode foils from the current collector may easily occur duringthe washing out of the plasticizer, which has unfavorable consequencesfor the cycle life and the impedance of a battery. It is thereforeproposed according to the present invention as an especially favorablesolution that electrochemical components be manufactured with thecurrent collectors manufactured according to the present invention,whose electrodes and separator were manufactured without a plasticizerthat has to be washed out.

The present invention shall be explained in greater detail below on thebasis of examples.

EXAMPLE 1 Copper Expanded Metal

A copper foil with a thickness of 50 μm was coated on both sides with acommercially available suspension EB012 from Acheson Colloids B.V. (athixotropic graphite suspension in a thermoplastic binder). To set theoptimal viscosity for the application, the solids content in thesuspension was reduced from 30% to 20% by adding water. Coating wascarried out on one side by means of a simple laminating roller first onthe front side and, in a second run, on the reverse side. The copperfoil was a commercially available standard foil for use in batteries.The wet layer thickness applied was approx. 20 μm at a feed rate of 2.5m/minute. Drying was carried out at approx. 80° C. The layer thicknessof the adhesion promoter layer was still 4 μm after drying. The foilthus coated was subsequently subjected to further processing intoexpanded metal. Stretching was set such that the short diagonal had alength of 1 mm and the long diagonal had a length of 2 mm. The materialobtained was free from separations and cracks in the metal and was ableto be subjected to further use at a rate of 100%.

COMPARISON EXAMPLE 1

Example 1 was repeated, and stretching was set such that the shortdiagonal had a length of 1.5 mm and the long diagonal had a length of 3mm. There were cracks in the product; it was flaked off in some areas.The reject was about 30% of the area.

COMPARISON EXAMPLE 1a

Example 1 was repeated such that the copper foil was first convertedinto expanded metal and this was coated as described. A large number ofcracked areas and areas with flaked-off coating were found on thematerial obtained in a non-uniform distribution. Only one of 6 batches(rolls) was suitable for use in such a way that it was able to be usedfor the further processing of the expanded metal into currentcollectors. On the whole, more than 50% of the area of the expandedmetal was damaged.

EXAMPLE 2 Aluminum Expanded Metal

An aluminum foil with a thickness of 50 μm was coated on both sides withthe above-mentioned, commercially available suspension EB012 fromAcheson Colloids B.V. To set the optimal viscosity for the application,the solids content in the suspension was reduced from 30% to 20% byadding water. Coating was carried out by means of a simple laminatingroller on one side, first on the front side and, in a second run, on thereverse side. The copper foil was a commercially available standard foilfor use in batteries. The wet layer thickness applied was approx. 20 μmat a feed rate of 2 m/minute. Drying was carried out at approx. 80° C.The layer thickness of the adhesion promoter layer was still 4 μm afterdrying. The foil thus coated was subsequently subjected to furtherprocessing into expanded metal. Stretching was set such that the shortdiagonal had a length of 1 mm and the long diagonal had a length of 2mm. The material obtained showed no separations and cracks in the metaland was able to be used further at a rate of 100%.

COMPARISON EXAMPLE 2

Example 2 was repeated, and stretching was set such that the shortdiagonal had a length of 1.5 mm and the long diagonal had a length of 3mm. The product had cracks in the coating; it was flaked off in someareas. The reject was about 25% of the area.

EXAMPLE 3 Anode Foil

To prepare an anode foil, 1.7 g of spheroidal graphite MCMB were mixedwith 0.1 g of conductive carbon black (acetylene black), 0.2 g ofpolyvinylidene fluoride, copolymer (PVDF-HFP) and 2 g of acetone andprocessed into a uniformly dispersed paste in a cutting mixer. Thispaste was subsequently applied to a glass plate to form a foil with adoctor plate. A self-supporting foil, which was removed from the glassplate, was left behind after the evaporation of the solvent. The layerthickness of the dried layer was approx. 100 μm.

EXAMPLE 4 Cathode Foil

Corresponding to the anode foil, a cathode foil of equal size wasprepared with the following composition: 3.6 g of LiCoO₂ were mixed with0.2 g of conductive carbon black (acetylene black) and 0.2 g of PVDF aswell as 4 g of acetone. Its layer thickness was likewise approx. 100 μm.

EXAMPLE 5 Separator Foil

1.5 g of a ceramic filler (lithium aluminum titanium phosphate)Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃ was processed with 0.5 g of PVDF-HFP and2.4 g of acetone as described in (3) to prepare a separator foil with athickness of about 50 μm.

EXAMPLE 6 Lamination

The electrode foils were laminated onto the particular collector gridsin a roll type laminator. The foils were preheated to 160° C. and thenlaminated under the roller with a pressing force of 236 kp. The feedrate was 40 mm/sec. Subsequent tape tests showed good adhesion of theparticular foils to the corresponding collector grids. The threeelements, namely, the anode with the copper collector grid, the cathodewith the aluminum collector grid and the separator foil, were laminatedtogether in a second lamination step. The force was 16 kp, likewise at alamination temperature of 160° C. and a feed rate of 20 mm/sec. Thedesign of the battery body is shown in FIGS. 1 and 2. FIG. 1 shows across section through a battery body, while FIG. 2 shows the top view ofa battery body. FIG. 1 shows the aluminum expanded metal (4) coated withadhesion promoter with the cathode foil (5) laminated to it and with theseparator foil (6). The counterelectrode consists of copper expandedmetal (8) coated with adhesion promoter with the anode foil (7)laminated to it. The aluminum expanded metal is seen in the top view inFIG. 4. Two contact tongues (9) for contacting the body after packagingin foil are led out to the side.

EXAMPLE 7 Manufacture of the Battery

The battery was introduced into a plastic-coated aluminum foil such thatelectric contacts were able to be led to the outside from the currentcollectors. A commercially available conducting salt solution LP30 wassubsequently introduced into the laminated foil composite by absorptionin a water-free protective gas atmosphere. The bag was then sealedhermetically. The battery was then formed and subsequently measuredelectrically. A good cycle life was found under a load with C rate. Thecurve is shown in FIG. 4. More than 80% of the initial capacity wasstill present after 300 charge/discharge cycles.

The relative capacity of the battery was compared to that of a batterywhose collector consisted of (error-free) coated expanded metalmanufactured in the conventional manner. As is apparent from FIG. 5, theperformance data of the two batteries are essentially identical. Theprocess according to the present invention consequently leads tocoatings of the same quality as in the case of expanded metals coated inthe usual manner.

1. Process for manufacturing expanded metal provided with a coating,comprising: applying the coating to a closed metal foil and convertingthe closed metal foil into expanded metal only after applying thecoating.
 2. Process in accordance with claim 1, wherein the coating is acoating that improves at least one of adhesiveness of the expanded metalto an electrode material and electron conductivity on a surface of theexpanded metal.
 3. Process in accordance with claim 1, wherein thecoating contains at least one of graphite, another carbon materialtogether with a binder that improves the adhesiveness and one of anorganic and inorganic-organic polymer, which is graphitized after theapplication to the metal.
 4. Process in accordance with claim 1, whereinthe metal comprises one of copper and aluminum.
 5. Process in accordancewith claim 1, wherein the metal foil is subjected to a corona dischargesurface treatment before it is coated.
 6. Process in accordance withclaim 1, wherein when the metal foil is converted into said expandedmetal, with a short diagonal length of up to 1 mm and a long diagonallength of up to 2 mm.
 7. Process in accordance with claim 1, wherein thecoating is applied by means of at least one of a printing technique,spin coating, rolling, application with a doctor blade, dip coating,electrostatic powder coating and by means of a plasma process. 8.Expanded metal provided with a coating, manufactured according to aprocess in accordance with claim
 1. 9. Expanded metal provided with acoating, obtained according to a process in accordance with claim
 1. 10.Expanded metal provided with a coating in accordance with claim
 2. 11.Expanded metal provided with a coating in accordance with claim
 3. 12.The method of claim 2, further comprising use of said expanded metal asa current collector associated with one of an anode foil and a cathodefoil.
 13. The method of claim 12, further comprising laminating togetherthe current collector in said anode foil and said cathode foil.
 14. Themethod of claim 12, wherein the anode foil and the cathode foil areprepared without using a plasticizing agent.
 15. The method of claim 2,further comprising using the expanded metal in an electrochemical cell,especially a battery.
 16. The method of claim 15, wherein the battery isa lithium battery.
 17. The method of claim 16, wherein the battery wasmanufactured according to a technique that does not require addition ofplasticizing agent and its subsequent washing out.